1. Field of the Invention
This invention relates to photonic integrated circuits, and more particularly to arrays of signal channels, each signal channel including a plurality of electro-optic elements for producing optical signals for transmission at high rates across a digital optical network infrastructure.
2. Description of the Related Art
Optical transmission networks are deployed for transporting data in long haul networks, metropolitan area networks, and other optical communication applications. More recently, advanced photonic integrated circuits have been designed and utilized to provide the necessary electrical-to-optical conversion necessary to realize such optical networks. In the simplest form, such photonic integrated circuits typically comprise one or more electro-optical elements in a signal channel which cooperate to provide a modulated optical output signal corresponding to an electrical data signal received for transmission across the optical network infrastructure. Such electro-optical elements may include, for example, laser sources, modulators, modulated laser sources, amplifiers and attenuators, among other elements. Exemplary photonic integrated circuits used as part of the transmitters and receivers of a digital optical transmission network can be found in U.S. Pat. No. 7,283,694, entitled “TRANSMITTER PHOTONIC INTEGRATED CIRCUITS (TXPIC) AND OPTICAL TRANSPORT NETWORKS EMPLOYING TXPICS,” and U.S. Pat. No. 7,116,851, entitled “AN OPTICAL SIGNAL RECEIVER, AN ASSOCIATED PHOTONIC INTEGRATED CIRCUIT (RxPIC), AND METHOD IMPROVING PERFORMANCE,” both of which are incorporated herein in their entirety by reference.
A digital optical transmission network is limited in performance due to several issues, including the optical signal-to-noise ratio and the signal quality or Q at both the optical transmitter and receiver. The Q of the optical receiver, that is the level of distortion in the optical signal which the optical receive can tolerate, is affected by factors, such as, for example, the power variations in the optical transmitter, wavelength dependent losses, and insertion losses in the transmitter and receiver. Moreover, improper handling of the circuits or systems related to the digital optical transmission network can result in further defects related to electrostatic discharge, and such defects further reduce the operating range of the corresponding electro-optic elements making up such circuits or systems. The accumulative effect of the foregoing is to limit the overall reach of the optical transmission system or, alternatively, to increase the cost of the system.
It is often desirable to place one or more light-absorbing electro-optic elements within the signal channel of the transmitter or receiver, for example to provide power monitoring or power equalization, or both. For power monitoring, such light-absorbing electro-optic elements can directly acquire data related to one or more of the above performance issues from light propagating along the optical path of the signal channel, and enhance the ability of the transmitter photonic integrated circuit to achieve higher levels of performance. Typically, for illustration purposes only, in the acquisition of information, the optical signal along the path of the signal channel in the transmitter is sensed and converted into a measurable quantity representative of the acquired information, for example a characteristic of the optical signal related to the aforementioned performance issues. For power equalization, such light-absorbing elements can be biased to absorb a known amount of optical power, thus reducing the amount of light which propagates along the remainder of optical path of the signal channel. In addition to the power monitoring described above, the light-absorbing element can be further biased to absorb more or less optical power, as desired, to provide a desired output power for that particular signal channel.
With the addition of such light-absorbing electro-optic elements, however, comes additional drawbacks. One drawback is the optical power loss, or insertion loss, associated with such electro-optic elements positioned along the optical path of the signal channel. As stated immediately above, information regarding the optical signal can be obtained, but in doing so, some amount of the optical signal is typically absorbed, or otherwise lost. Compensation for such loss can be achieved through further amplification, however such compensation comes with higher power requirements, eventually realized in increasing operating costs. Additionally, at high data rates, the portion of the optical signal which is absorbed by such light-absorbing electro-optic elements positioned within a signal channel results in a corresponding high speed pulsing photocurrent which then acts to further bias the element. Such self-biasing detrimentally affects the signal quality or Q of the optical signal propagating down the signal channel.
Another drawback is the radio frequency (RF) interference associated with such light-absorbing electro-optic elements. For example, with respect to monitoring elements, as the optical signal is converted into a measurable quantity, an electrical signal having oscillatory characteristics similar to the optical signal itself may be generated. Such oscillating electrical signals can be electrically coupled to other electro-optic elements in that signal channel, or another signal channel in close proximity, resulting in noise or undesirable crosstalk and, ultimately, optical signal degradation. The self-biasing affects due to high speed optical signals traveling through light-absorbing electro-optic elements may also lead to undesirable crosstalk and contribute to such signal degradation.
Still another drawback of having light-absorbing electro-optic elements positioned within a signal channel is electrostatic discharge susceptibility. For example, electrical signals correspond to one or more characteristics related to the optical signal propagating in a signal channel, or which are provided to allow for power manipulation in an optical signal, typically must be processed. Such processing of the electrical signals may be performed in a circuit spaced from the photonic integrated circuit itself. With an increase in the number of such electro-optic elements present in the signal channel, therefore, can often result in higher pin counts at the point where the photonic integrated circuit interfaces with additional external circuitry, for example circuitry which will process the electrical signals from the light-absorbing electro-optic devices. Improper handling of such devices, during manufacture or repair for example, can result in failure or impairment of the associated electro-optic devices resulting in inoperative transmission systems along the signal channel, impaired signal quality, or impaired overall performance.
What is needed is a photonic integrated circuit which includes one or more light-absorbing electro-optic elements and supporting circuitry configured to obtain or manipulate a characteristic related to the optical signal propagating along a signal channel while reducing performance losses associated with such light absorbing devices. Furthermore, what is needed is a photonic integrated circuit which allows for smaller sized performance monitoring electro-optic elements resulting in lower optical signal power loss, and reducing the operating costs of such systems employing such photonic integrated circuits. Further, a photonic integrated circuit is needed which minimizes the RF crosstalk related to the addition of certain performance monitoring electro-optic elements. In addition, a photonic integrated circuit is needed which includes performance monitoring electro-optic elements having a decreased susceptibility to electrostatic discharge.